1. Technical Field
The present invention relates to a method for arraying nano material, and more particularly, to a method for arraying nano material in a uniform direction, and to a method for fabricating a liquid crystal display device using the same.
2. Description of the Related Art
With the rapid development of information communication technology, requirements on display devices are increasing. To meet the requirements, display devices having thin film transistors (TFTs) have been introduced which can provide excellent characteristics in views of resolution and implementation of moving picture. The TFTs can reduce power consumption of the display devices and are also advantageous to fabricate large-sized display devices.
A method of forming a related art TFT will now be described in detail with reference to the accompanying drawings.
FIGS. 1A and 1B are schematic views of a related art TFT. Specifically, FIG. 1A is a plan view of the related art TFT and FIG. 1B is a sectional view taken along line I-I′ of FIG. 1A.
Referring to FIGS. 1A and 1B, a substrate 10 is prepared. A conductive material is deposited on the substrate 10 and is then patterned to form a gate electrode 20. A gate insulating layer 30 is formed on an entire surface of the substrate 10 where the gate electrode 20 is formed. A semiconductor layer 40 is formed on the gate insulating layer 30 corresponding to the gate electrode 20. A metal layer is deposited on the semiconductor layer 40 and is then patterned to form source/drain electrodes 50a and 50b. Consequently, the TFT is formed through these procedures.
The semiconductor layer 40 may be an inorganic semiconductor layer or an organic semiconductor layer. The inorganic semiconductor layer may be formed of amorphous silicon (a-Si) or polysilicon (p-Si). For example, as illustrated in FIGS. 1A and 1B, the inorganic semiconductor layer may include an active layer 40a and an ohmic contact layer 40b formed by sequentially depositing and patterning amorphous silicon and P-type or N-type doped amorphous silicon. Also, the organic semiconductor layer may be formed of polysilicon that is made by crystallizing amorphous silicon.
In forming the inorganic semiconductor layer, it is necessary to form a thin film using an expensive deposition apparatus and perform an etching process, resulting in the increase of the manufacturing cost. Specifically, if the inorganic semiconductor layer is formed of the polysilicon having higher carrier mobility than that of the amorphous silicon, device characteristics can be improved. In this case, however, since the crystallization process is performed at a high temperature, a lot of problems may be caused. In addition, high technology is required to form a uniform polysilicon layer. Therefore, there is a great difficulty in forming the inorganic semiconductor layer 40.
On the contrary, the organic semiconductor layer can be easily formed and also used in flexible display devices. The organic semiconductor layer, however, has lower carrier mobility than that of the inorganic semiconductor layer. Therefore, if an on-current level is increased, the size of the TFT is also increased. If the size of the TFT in the display device is increased, a region occupied by the pixel electrode in the unit pixel is reduced. Consequently, an aperture ratio is reduced and a cost is increased.
To solve these problems, display devices using nano transistors have been introduced.
The nano transistor includes an active layer and source/drain electrodes. The active layer is formed of nano material such as nanowire and nanotube, and the source/drain electrodes are arranged spaced apart from each other on the active layer. The active layer can be easily formed using a wet etching process, instead of a deposition process. Also, device characteristics are excellent because of the use of nano material.
It is preferable that the nano material should be arranged in a uniform direction. In this case, the carrier mobility of the active layer formed of the nano material can be increased. Also, in forming a plurality of nano transistors at the same time, the respective nano transistors may be formed to have the same characteristics. Also, in the display devices using the nano transistors, it is possible to secure uniform electrical characteristics of the nano transistors, thereby obtaining uniform image quality of the display devices.
One of methods for arraying the nano material in a uniform direction is a Langmuir-Blodgett method. According to the Langmuir-Blodgett method, a substrate where a self assembly monolayer is formed is soaked in a solution where nano material is dispersed, and the nano material is adsorbed on the substrate. However, the Langmuir-Blodgett method is not suitable for mass production and its stabilization is degraded.
According to another method, trenches having a uniform direction are formed on a substrate, where nano material will be formed, and nano material is then inserted into the trenches. However, this method is difficult to form nano-sized trenches. Also, it is difficult to insert nano material into the nano-sized trenches, causing device failure.